[V3] Add tox (#1641)

* Configure tox environments for install, dev install and docs build

* Configure Travis to run tox

* Use 3.5.1 since it's our minimum supported version

* Bump lower travis build version to 3.5.2

Turns out a dependency is incompatible with 3.5.1.

* Modify Travis config to install from pipenv

* Try without skipping the lock

* Try without pip cache

* D the dev install with pipenv

* See if adding the pip cache back in breaks

* Remove the development installation

It doesn't really make any sense considering we already should be installed in develop mode, as does Travis.

* Oops, tox should go under dev packages

* Do black --check with tox

* Uncache pip again...

* Try a build matrix, and try ignoring virtualenvs

* Activate pipenv shell on travis

* Try installing prereleases

* Try the build matrix like this

* Try exclusion

* Upgrade pip

* Try this environment marker

* Back to stages...

* Try run over shell

* Try skipping the lock again

* This'll be faster but probably ignore 3.5

Because Travis

* Just manually list sources for black to check

* Magic?

* What if I told you...

That this worked perfectly on Tobotimus/Red-DiscordBot@test_travis_matrix

* It couldn't possibly be this easy

* Let's add some comments to be nice

* Let's change back to trusty just in case the stages fuck up

* Add another comment because why not

* Let's try caching pip one more time

* We don't need to whitelist these

setup.py

@@ -115,5 +115,6 @@ setup(
         "mongo": ["motor"],
         "docs": ["sphinx>=1.7", "sphinxcontrib-asyncio", "sphinx_rtd_theme"],
         "voice": ["red-lavalink>=0.0.4"],
+        "style": ["black"],
     },
 )